Research of control strategies of MMC based on full bridge sub-modules

Although full bridge sub-module topologies require more power devices than half bridge topology, it can offer flexible control of positive, negative, and zero voltage output characteristics. Here, full-bridge MMC with three kinds of operating modes as positive, negative voltage output with low voltage operation, and DC-link fault current suppression with zero voltage output are studied. To deal with the problem of insulation reduction when long-distance overhead line is applied and reduced DC voltage operation is required when LCC is connected with MMC, the effect of constant power and reduced power transmission on the storage capacitor voltage balance are studied. What is more, the zero voltage output characteristic is also studied not only to avoid the problem that the sub-module capacitor voltage may diverge due to long time blocking converter during DC line fault but also to supply reactive power for AC grid. Control strategies are designed under different operating modes to achieve full bridge MMC flexible operation. Finally, a simulation model is built in PSCAD/EMTDC to simulate the output characteristics of full-bridge MMC in different operation modes